Immature proglottids are those in which the reproductive structures are just starting to develop, but are not yet functional. In these ones, the genital pores have formed on the margins, and the darkly staining structure proceeding towards the midline is the cirrus which is just starting to develop. The numerous small darkly stained masses found throughout the proglottid are rudimentary testes, and the dark mass near the posterior midline is the developing vitellarium.